Photophysical and Structural Properties of Cyanoruthenate

Aug 22, 2007 - These complexes are of interest both for their photophysical properties and ability to act as sensitizers, associated with strong MLCT ...
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Photophysical and Structural Properties of Cyanoruthenate Complexes of Hexaazatriphenylene Juan-Manuel Herrera,† Simon J. A. Pope,‡ Anthony J. H. M. Meijer,† Timothy L. Easun,† Harry Adams,† Wassim Z. Alsindi,§ Xue-Zhong Sun,§ Michael W. George,§ Stephen Faulkner,| and Michael D. Ward*,† Contribution from the Department of Chemistry, UniVersity of Sheffield, Sheffield S3 7HF, U.K., School of Chemistry, Main Building, Cardiff UniVersity, Cardiff CF10 3AT, U.K., School of Chemistry, UniVersity of Nottingham, UniVersity Park, Nottingham NG7 2RD, U.K., and Department of Cmemistry, UniVersity of Manchester, Oxford Road, Manchester M13 9PL, U.K. Received April 30, 2007; Revised Manuscript Received July 4, 2007; E-mail: [email protected]

Abstract: The tritopic bridging ligand hexaazatriphenylene (HAT) has been used to prepare the mono-, di-, and trinuclear cyanoruthenate complexes [Ru(CN)4(HAT)]2- ([1]2-), [{Ru(CN)4}2(µ2-HAT)]4- ([2]4-), and [{Ru(CN)4}3(µ3-HAT)]6- ([3]6-). These complexes are of interest both for their photophysical properties and ability to act as sensitizers, associated with strong MLCT absorptions; and their structural properties, with up to 12 externally directed cyanide ligands at a single “node” for preparation of coordination networks. The complexes are strongly solvatochromic, with broad and intense MLCT absorption manifolds arising from the presence of low-lying π* orbitals on the HAT ligand, as confirmed by DFT calculations; in aprotic solvents [3]6- is a panchromatic absorber of visible light. Although nonluminescent in fluid solution, the lowest MLCT excited states have lifetimes in D2O of tens of nanoseconds and could be detected by timeresolved IR spectrosocopy. For dinuclear [2]4- and trinuclear [3]6- the TRIR spectra are indicative of asymmetric MLCT excited states containing distinct Ru(III) and Ru(II) centers on the IR time scale. The complexes show red 3MLCT luminescence as solids and in EtOH/MeOH glass at 77 K. Ln(III) salts of [1]2-, [2]4-, and [3]6- form infinite coordination networks based on Ru-CN-Ln bridges with a range of one-, two-, and three-dimensional polymeric structures. In the Yb(III) and Nd(III) salts of [3]6- the complex anion forms an 8-connected node. Whereas all of the Gd(III) salts show strong 3MLCT luminescence in the solid state, the Ru-based emission in the Nd(III) and Yb(III) analogues is substantially quenched by Ru f Ln photoinduced energy transfer, which results in sensitized near-infrared luminescence from Yb(III) and Nd(III).

Introduction

The complex [Ru(bipy)(CN)4]2, first reported by Bignozzi and Scandola in 1986, has a particularly attractive set of properties which make it of interest in the general area of syntheses and photophysical properties of supramolecular assemblies.1-3 Like its better-known cousin [Ru(bipy)3]2+ it has a luminescent Ru f bipy 3MLCT excited state, which is relatively long-lived (≈100 ns) in protic solvents. In addition to this moreover it has several other advantages from both a synthetic and a spectroscopic viewpoint. From the structural point of view, it is exceptionally easy to incorporate into supramolecular assemblies because the cyanide groups can participate in either coordination to additional metal ions4-6 or †

University of Sheffield. University of Cardiff. University of Nottingham. | University of Manchester. ‡ §

(1) (a) Bignozzi, C. A.; Chiorboli, C.; Indelli, M. T.; Scandola, M. A. R.; Varani, G.; Scandola, F. J. Am. Chem. Soc. 1986, 108, 7872. (b) Habib Jiwan, J. L.; Wegewijs, B.; Indelli, M. T.; Scandola, F.; Braslavsky, S. E. Recl. TraV. Chim. Pays-Bas 1995, 114, 542. (2) Ward, M. D. Coord. Chem. ReV. 2006, 250, 3128. (3) Timpson, C. J.; Bignozzi, C. A.; Sullivan, B. P.; Kober, E. M.; Meyer, T. J. J. Phys. Chem. 1996, 100, 2915. 10.1021/ja072672w CCC: $37.00 © 2007 American Chemical Society

in hydrogen-bonding with molecules containing acidic protons,7-10 and both of these mechanisms have been used to prepare noncovalently linked assemblies in which [Ru(bipy)(CN)4]2acts as an energy donor or acceptor in its excited state.2 In particular we have prepared cyanide-bridged coordination networks combining [Ru(bipy)(CN)4]2- as a visible lightabsorbing sensitizer unit with lanthanide(III) ions such as Nd(III), Er(III), Pr(III), and Yb(III) as energy-acceptors, allowing generation of sensitized near-infrared luminescence from these (4) (a) Davies, G. M.; Pope, S. J. A.; Adams, H.; Faulkner, S.; Ward, M. D. Inorg. Chem. 2005, 44, 4656. (b) Miller, T. A.; Jeffery, J. C.; Ward, M. D.; Adams, H.; Pope, S. J. A.; Faulkner, S. Dalton Trans. 2004, 1524. (c) Baca, S. G.; Adams, H.; Ward, M. D. Cryst. Eng. Commun. 2006, 88, 635. (5) (a) Herrera, J.-M.; Pope, S. J. A.; Adams, H.; Faulkner, S.; Ward, M. D. Inorg. Chem. 2006, 45, 3895. (b) Herrera, J.-M.; Baca, S.; Ward, M. D. Polyhedron 2006, 25, 869. (6) Lazarides, T.; Easun, T. L.; Veyne-Marti, C.; Alsindi, W. Z.; George, M. W.; Deppermann, N.; Hunter, C. A.; Adams, H.; Ward, M. D. J. Am. Chem. Soc. 2007, 129, 4014. (7) Derossi, S.; Adams, H.; Ward, M. D. Dalton Trans. 2007, 33. (8) Simpson, N. R. M.; Ward, M. D.; Morales, A. F.; Ventura, B.; Barigelletti, F. J. Chem. Soc., Dalton Trans. 2002, 2455. (9) Pina, F.; Parola, A. J. Coord. Chem. ReV. 1999, 185-186, 149. (10) Bergamini, G.; Saudan, C.; Ceroni, P.; Maestri, M.; Balzani, V.; Gorka, M.; Lee, S.-K.; van Heyst, J.; Vo¨gtle, F. J. Am. Chem. Soc. 2004, 126, 16466. J. AM. CHEM. SOC. 2007, 129, 11491-11504

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Ln(III) centers following d f f energy-transfer.4 From the spectroscopic point of view, [Ru(bipy)(CN)4]2- is strongly solvatochromic, with its 1MLCT and 3MLCT levels and Ru(II)/Ru(III) redox potential varying over a wide range in different solvents due to different degrees of hydrogen-bonding between the cyanide lone pairs and the solvent: the 3MLCT emission maximum varies from 610 nm in water to ≈800 nm in non H-bonding solvents such as MeCN and dmf.1-3 Interaction of the cyanide ligands with additional metal cations in solution has an even stronger effect, with “metallochromism” allowing the 3MLCT energy to be varied over a range of ca. 6000 cm-1.6 Thus, the ability of [Ru(bipy)(CN)4]2- to act as an energy donor in its excited state can be tuned by varying the solvent composition or by addition of group 1A or 2A metal salts to the solvent, which has been exploited as the basis of interesting switching effects such as reversal of the direction of energy transfer in a dyad.6,11 Finally, the cyanide group provides a convenient spectroscopic handle for monitoring the excited state using transient infrared spectroscopy since the position of the CN vibrations is sensitive to the electron distribution in the complex.12,13 Higher nuclearity analogues of [Ru(bipy)(CN)4]2- have been scarcely studied and have considerable appeal because many of the above advantages will be amplified. A larger number of cyanide groups present in a single molecule will allow preparation of structurally novel luminescent materials with a higher degree of connectivity to a single node, and should also allow for a greater degree of modulation of the spectroscopic properties via solvatochromism and metallochromism effects, since these depend on the number of cyanide sites available.3 We recently prepared dinuclear [{Ru(CN)4}2(µ-bpym)]4- (bpym ) 2,2′bipyrimidine) and used it as the basis of coordination networks with lanthanide cations.5 Up to six of the eight cyanide groups were found to be involved in bridging interactions to Ln(III) centers, and picosecond time-resolved IR studies showed that it has an asymmetric, valence-trapped 3MLCT excited state [(NC)4RuII(bpym•-)RuIII(CN)4]4-.13 In this paper, we describe the syntheses, spectroscopic, and some structural properties of the series of complexes [Ru(CN)4(HAT)]2- ([1]2-), [{Ru(CN)4}2(µ2-HAT)]4- ([2]4-), and [{Ru(CN)4}3(µ3-HAT)]6- ([3]6-) (Scheme 1) in which one, two, or three {Ru(CN)4}2- units are connected to the bidentate sites of the tritopic ligand hexaazatriphenylene (HAT).14 These complexes prove to have some very interesting properties. The low energy of the LUMO of the extended aromatic bridging ligand and the presence of up to three Ru(II) centers means that the 1MLCT absorptions are low energy and intense; there are obvious parallels with Ru(II) and Os(II) complexes of other highly extended aromatic diimine-based ligands such as eilatin/ dibenzoeilatin15 and the “heterosuperbenzenes”16 which show (11) (a) Simpson, N. R. M.; Ward, M. D.; Morales, A. F.; Barigelletti, F. J. Chem. Soc., Dalton Trans. 2002, 2449. (b) Indelli, M. T.; Ghirotti, M.; Prodi, A.; Chiorboli, C.; Scandola, F.; McClenaghan, N. D.; Puntoriero, F.; Campagna, S. Inorg. Chem. 2003, 42, 5489. (12) Adams, H.; Alsindi, W.; Davies, G. M.; Duriska, M. B.; Easun, T. L.; Fenton, H.; Herrera, J.-M.; George, M. W.; Ronayne, K. L.; Sun, X.-Z.; Towrie, M.; Ward, M. D. Dalton Trans. 2006, 39. (13) Alsindi, W. Z.; Easun, T. L.; Sun, X.-Z.; Ronayne, K. L.; Towrie, M.; Herrera, J.-M.; George, M. W.; Ward, M. D. Inorg. Chem. 2007, 46, 3696. (14) (a) Kitagawa, S.; Masaoka, S. Coord. Chem. ReV. 2003, 246, 73. (b) Patra, S.; Sarkar, B.; Ghumaan, S.; Fiedler, J.; Kaim, W.; Lahiri, G. K. Dalton Trans. 2004, 754. (c) D’Alessandro, D.; Keene, F. R. Chem.sEur. J. 2005, 11, 3679. (d) Leveque, J.; Moucheron, C.; Kirsch-De Mesmaeker, A.; Loiseau, F.; Serroni, S.; Puntoriero, F.; Campagna, S.; Nierengarten, H.; van Dorsselaer, A. Chem. Commun. 2004, 878. 11492 J. AM. CHEM. SOC.

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Scheme 1

unusually low energy absorption and emission bands. The presence of up to 12 cyanide groups which can interact with solvent molecules is expected to provide a high degree of solvatochromism. In addition the potential connectivity of up to 12 provided by the externally directed cyanide binding sites of [{Ru(CN)4}3(µ3-HAT)]6- is unprecedented in cyanide-based coordination networks. The highest connectivity observed so far at a single cyanometalate node is eight, in a network in which all eight cyanides of [W(CN)8]4- are coordinated to Co(II) ions,17 although this is rare: usually octacyanometalates of Mo and W connect to six or less other metal cations.18 Accordingly this series of high-nuclearity cyanoruthenates, with up to 12 externally directed cyanide ligands, is expected to be of interest equally for its structural properties and its photophysical properties, and we describe here an extensive investigation into both aspects of the chemistry of [1]2-, [2]4-, and [3]6-. A preliminary Communication describing a small part of this work has been published recently.19 Results and Discussion

Synthesis and Characterization. Reaction of the tritopic ligand HAT with 3.6 equiv of K4[Ru(CN)6]·3H2O in aqueous acid (pH 1.3) at reflux for 48 h afforded a deep violet solution from which the complexes [1]2- (orange-red), [2]4- (deep red), and [3]6- (violet) could be isolated as their sodium salts by ionexchange chromatography on Sephadex QAE-25 eluting with aqueous NaI. In this form the complexes are isolated as their sodium salts and are water-soluble; to render them soluble in organic solvents, the complexes were converted to their PPN+ salts (PPN+ is Ph3PdNdPPh3+), which are soluble in polar organic solvents such as MeCN and dmf.20 The complexes were satisfactorily characterized on the basis of elemental analyses, negative-ion electrospray mass spectra, and 1H NMR spectra. (15) (a) Bergman, S. D.; Gut, D.; Kol, M.; Sabatini, C.; Barbieri, A.; Barigelletti, F. Inorg. Chem. 2005, 44, 7943. (b) Bergman, S. D.; Goldberg, I.; Barbieri, A.; Kol, M. Inorg. Chem. 2005, 44, 2513. (16) (a) Draper, S. M.; Gregg, D. J.; Schofield, E. R.; Browne, W. R.; Duati, M.; Vos, J. G.; Passaniti, P. J. Am. Chem. Soc. 2004, 126, 8694. (b) Gregg, D. J.; Bothe, E.; Hofer, P.; Passaniti, P.; Draper, S. M. Inorg. Chem. 2005, 44, 5654. (c) Gregg, D. J.; Fitchett, C. M.; Draper, S. M. Chem. Commun. 2006, 3090. (17) Herrera, J.-M.; Bleuzen, A.; Dromze´e, Y.; Julve, M.; Lloret, F.; Verdaguer, M. Inorg. Chem. 2003, 42, 7052. (18) Sieklucka, B.; Podgajny, R.; Przychodzen, P.; Korzeniak, T. Coord. Chem. ReV. 2005, 249, 2203. (19) Herrera, J.-M.; Ward, M. D.; Adams, H.; Pope, S. J. A.; Faulkner, S. Chem. Commun. 2006, 1851. (20) Evju, J. K.; Mann, K. R. Chem. Mater. 1999, 11, 1425.

Cyanoruthenate Complexes of Hexaazatriphenylene

ARTICLES Table 1. Selected Bond Distances in the Structure of Na2[1]·4H2Oa

Ru(1)-C(35) Ru(1)-C(33) Ru(1)-C(37) Ru(1)-C(31) Ru(1)-N(11) Ru(1)-N(1) Na(1)-O(12) Na(1)-O(11) Na(1)-N(24)#3 Na(1)-N(32)#4 Na(1)-N(38)

1.981(9) 1.988(9) 2.043(9) 2.051(9) 2.101(7) 2.112(7) 2.343(8) 2.347(8) 2.431(8) 2.465(9) 2.600(9)

Na(1)-N(38) Na(1)-N(14)#3 Na(1)-Na(2)#1 Na(2)-O(22) Na(2)-O(21) Na(2)-N(21) Na(2)-N(38)#1 Na(2)-N(32)#2 Na(2)-N(4) Na(2)-Na(1)#1

2.600(9) 2.967(8) 3.907(6) 2.295(8) 2.336(8) 2.459(8) 2.514(9) 2.691(9) 2.881(9) 3.907(6)

a Symmetry transformations used to generate equivalent atoms: (#1) -x + 1, -y, -z + 1; (#2) -x + 1/2, y - 1/2, -z + 3/2; (#3) -x + 1/2, y + 1/2, -z + 1/2; (#4) x + 1/2, -y + 1/2, z - 1/2.

Figure 1. Asymmetric unit of the structure of Na2[1]·4H2O, with additional atoms from adjacent asymmetric units to complete the coordination spheres around Na(1) and Na(2) included.

Figure 3. Electronic absorption spectra of [1]2- (black), [2]4- (red), and [3]6- (blue), in water (solid lines) and dmf (dashed lines).

Figure 2. Network structure of Na2[1]·4H2O (Na, green; Ru, orange; O, red; N, blue; C, black).

In particular, the 1H NMR spectra for [1]2- and [2]4- show three signals (two doublets and a singlet) for the HAT ligand, as expected for C2V symmetry; for the more highly symmetric complex [3]6- a single resonance is seen for the HAT ligand. Slow evaporation of an aqueous solution of Na2[1] afforded X-ray-quality crystals of Na2[1]·4H2O whose structure is shown in Figures 1 and 2 (see also Table 1). The structure of the [Ru(HAT)(CN)4]2- anionic fragment is unremarkable, with octahedral coordination around the Ru(II) center and essentially linear cyanide ligands (Ru-C-N angles between 177 and 178°). Coordination of both cyanide groups and diimine sites of the HAT ligand to the Na+ counterions results in formation of a coordination network. Na(1) and Na(2) are both six-coordinate, from two water ligands, two cyanides (from two different [1]2units), and a diimine site from a third [1]2- unit. Thus, all three bidentate sites of the HAT ligand are occupied, one by the {Ru(CN)4}2- unit and the other two by Na+ cations, although the interaction between the bidentate HAT sites and each Na+ ion is markedly asymmetric, with one short Na-N bond [Na(1)-N(24), 2.43 Å; Na(2)-N(21), 2.46 Å] and one long bond [Na(1)-N(14), 2.97 Å; Na(2)-N(4), 2.88 Å]. Na(1) and Na(2) are close together (3.91 Å) and bridged by the two cyanide ligands N(32) and N(38), which are the axial pair from [1]2-. The equatorial pair of cyanide ligands [containing N(34) and

N(36)] are not involved in coordination to Na+ ions but are involved in OH···NC hydrogen-bonding to water ligands on the Na+ ions, with two nonbonded O···N separations of 900 nm. There are therefore several closely spaced 1MLCT components which are not resolved in water and give an apparently single absorption band, but these spread out in solvents with a lower Gutmann acceptor number (see below for a computational analysis). For dinuclear [2]4- the presence of at least two components to the 1MLCT absorption manifold is obvious even in water, where the spectrum displays a maximum at 478 nm and a shoulder of nearly the same intensity at 516 nm; in dmf the most intense peak is red-shifted to 596 nm, but there are two intense lower energy shoulders at ≈700 and 825 nm which result in the absorption spectrum extending out to ≈1000 nm. The red-shift in the most intense absorption component between water and dmf is 4140 cm-1, which is actually less than for [1]2- despite the larger number of cyanide ligands, but this obviously does not tell the whole story as the lower energy absorption components are much more strongly red-shifted (ca. 7200 cm-1 for the lowest energy feature). The same general behavior occurs for [3]6-. The main 1MLCT absorption band in water is at 524 nm and is rather broad, indicative of two or more closely spaced components; in dmf these have spread out to the extent that four components are clearly visible, with the lowest energy featuresa shoulder at ca. 930 nmsextending the low-energy tail out to ca. 1050 nm. The red-shift in the absorption maximum between water and dmf is 4630 cm-1, comparable to that seen for [1]2- and [2]4-, but again it is the lower energy features that are far more dramatically red-shifted to the extent of >8000 cm-1 for the lowest energy feature in the spectrum of [3]6-. This results in panchromatic absorbance of light all the way across the visible region and extending well (21) Gutmann, V.; Resch, G.; Linert, W. Coord. Chem. ReV. 1982, 43, 133. 11494 J. AM. CHEM. SOC.

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Figure 4. HOMO and LUMO of the hydrated cluster [3]6-·12H2O calculated using DFT methods (see main text).

into the near-infrared. This is a particularly desirable characteristic for complexes used in dye-sensitized solar cells as it results in effective photocurrent generation throughout the solar emission spectrum, and considerable synthetic effort has gone into the design and synthesis of complexes which display panchromatic absorption in this way.22 Indeed, in dmf complexes [2]4- and [3]6- have light-absorbing characteristics superior to those of the [Ru(terpy)(NCS)3]- derivatives used as “black” dyes in solar cells, with much higher extinction coefficients and a longer low-energy absorption tail.22c,e Molecular Orbital Calculations. DFT calculations using GAUSSIAN were performed on [3]6- to determine the nature of the frontier orbitals and rationalize the electronic spectra. The HOMO and LUMO calculated in the presence of specific hydrogen-bonding interactions to water (modeled by one CN· ··HOH interaction/cyanide, with an N···O separation after geometry optimization of 2.7 Å) are shown in Figure 4. The HOMO is largely based on the Ru d(π) orbital set (with a small contribution from the cyanides), and the LUMO is localized on the HAT ligand. Thus, the lowest-energy electronic transition can be assigned with confidence as 1MLCT. What differentiates this complex from simple mononuclear species such as [Ru(bipy)(CN)4]- is the presence of a manifold of six closely spaced π* orbitals on the HAT ligand and likewise six closely (22) (a) Yanagida, M.; Yamaguchi, T.; Kurashige, M.; Hara, K.; Katoh, R.; Sugihara, H.; Arakawa, H. Inorg. Chem. 2003, 42, 7921. (b) Islam, A.; Sugihara, H.; Yanagida, M.; Hara, K.; Fujihashi, G.; Tachibana, Y.; Katoh, R.; Murata, S.; Arakawa, H. New J. Chem. 2002, 26, 966. (c) Nazeeruddin, M. K.; Pechy, P.; Renouard, T.; Zakeeruddin, S. M.; Humphry-Baker, R.; Comte, P.; Liska, P.; Cevey, L.; Costa, E.; Shklover, V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Gra¨tzel, M. J. Am. Chem. Soc. 2001, 123, 1613. (d) Gra¨tzel, M. Inorg. Chem. 2005, 44, 6841. (e) Ghosh, S.; Chaitanya, G. K.; Bhanuprakash, K.; Nazeeruddin, M. K.; Gra¨tzel, M.; Reddy, P. Y. Inorg. Chem. 2006, 45, 7600.

Cyanoruthenate Complexes of Hexaazatriphenylene

spaced occupied orbitals of principally Ru-centered 4d character. Thus, a large collection of closely spaced 1MLCT absorptions is expected, as we observe. To investigate this further we calculated the excitation energies for [1]2-, [2]4-, and [3]6- in DMF and in H2O using the TD-DFT method. TD-DFT has well-publicized problems with MLCT states, and therefore, we cannot expect a quantitatively correct answer.23 However, because the errors for each compound will be similar, we can use these results to get a qualitative understanding of the photophysics. The lowest 10 excitation energies for each compound are given in Table S1 (Supporting Information); the coordinates for the calculated energy-minimized structures used in the calculations are in Table S2. It is clear from the calculations that the excitation energies for the compounds dissolved in DMF are generally lower than for the same compounds dissolved in water, in agreement with the experimental data. It is also apparent that the calculated excitation energies decrease with increasing number of Ru atoms in the complex, again in agreement with experimental data (see Table 2). Thus, for the series [1]2-, [2]4-, and [3]6-, the lowest energy absorptions are calculated as 738, 761, and 792 nm, respectively, in dmf and 664, 700, and 741 nm, respectively, in water. Note that, due to the fact that no symmetry was imposed on the calculations, expected degeneracies in the excitation energies will be slightly lifted due to numerical rounding errors. The effect of changing solvent from water to dmf in these calculations is not as large as we observed experimentally. For example, the lowest 10 MLCT excitations for [3]6- are calculated by this method to lie in the range 701-792 nm in dmf and between 663 and 741 nm in water, giving an average blue-shift of ca. 850 cm-1 in water, much less than actually observed (see previous section). This is probably partially due to the problems with using TD-DFT for charge-transfer states as indicated above; in addition our allowance of one hydrogenbonded water molecule/cyanide ligand may be too modest, with much more extensive aquation being present in reality24 which would result in a bigger difference between absorption energies in water and DMF. We note also that although DFT gives good geometries (even for van der Waals bonded systems), it generally significantly underestimates the van der Waals binding energy and will therefore underestimate the influence of the weakly bound water molecules on the electronic wave function of the complex. To conclude this section, we emphasize that quantitative conclusions about the electronic spectra are not appropriate given the known difficulties in calculations of charge-transfer transitions using TD-DFT.23 However the calculations do confirm that (i) a manifold of several closely spaced 1MLCT absorptions is expected due to the presence of several low-lying π* orbitals on the HAT ligand, (ii) the absorption manifold should be redshifted as the number of metal centers increases, with [3]6having the lowest energy MLCT absorptions, and (iii) these MLCT absorptions should be red-shifted in the absence of hydrogen-bonding interactions, also as observed. (23) (a) Dreeuw, A.; Head-Gordon, M. Chem. ReV. 2005, 105, 4009. (b) Dreeuw, A.; Weisman, J.; Head-Gordon, M. J Chem. Phys. 2003, 119, 2943. (c) Tozer, D. J. Chem. Phys. 2003, 119, 12697. (d) Tozer, D. J.; Amos, R. D.; Handy, N. C.; Roos, B. O.; Serrano-Andre´s, L. Mol. Phys. 1999, 97, 859 and references therein. (24) Megyes, T.; Schubert, G.; Kova´cs, M.; Radnai, T.; Gro´sz, T.; Bako´, I.; Pa´pai, I.; Horva´th, A. J. Phys. Chem. A 2003, 107, 9903.

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Figure 5. Normalized luminescence spectra (corrected for detector sensitivity) of [1]2- (green), [2]4- (blue), and [3]6- (red), as their Na+ salts in EtOH/MeOH (4:1, v/v) glass at 77 K.

Figure 6. Time-resolved IR difference spectra of [1]2- (top) and [2]4(bottom) in D2O each measured at several time intervals after excitation. The key shows the time delay after excitation (in nanoseconds) at which each spectrum was recorded.

Excited-State Properties in Solution. No luminescence could be detected from these complexes in aqueous solution. Given the general pattern for [Ru(bipy)(CN)4]2- derivatives that in less Lewis-acidic solvents the 3MLCT energy is reduced in energy compared to water and the luminescence is weaker and shorter-lived,1-3 we did not expect (and did not observe) luminescence in any other solvents either at room temperature. However in a 4:1 EtOH/MeOH glass at 77 K, weak luminescence bands were observed in each case with maxima (corrected for the detector response) at 739, 748, and 770 nm for Na2[1], Na4 [2], and Na6[3], respectively (Figure 5), which gives 3MLCT energies of 13 500, 13 400, and 13 000 cm-1, respectively, under these conditions. We examined the excited-state behavior of the complexes in solution by time-resolved infrared spectroscopy, which allows non- or poorly luminescent excited states to be observed by changes in their IR absorption spectra. Figure 6a shows the transient IR difference spectra in the ν(CN) region of Na2[1] in D2O with different time delays between pump and probe. The results are in agreement with what we have observed for other mononuclear complexes of the [Ru(diimine)(CN)4]2- type.12,13,25 (25) Encinas, S.; Morales, A. F.; Barigelletti, F.; Barthram, A. M.; White, C. M.; Couchman, S. M.; Jeffery, J. C.; Ward, M. D.; Grills, D. C.; George, M. W. J. Chem. Soc., Dalton Trans. 2001, 3312. J. AM. CHEM. SOC.

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The strongest ground-state ν(CN) band at ca. 2066 cm-1 is bleached, and a weak transient band to higher energy (ca. 2110 cm-1) is formed, both returning to the baseline on the same time scale with a lifetime of 44 ns. The transient band corresponds to the ν(CN) vibration in the excited state and is at higher energy because the MLCT process results in transient oxidation of the metal center from Ru(II) to Ru(III), which diminishes the Ru[d(π)] f CN(π*) back-bonding and results in the C-N bond being strengthened. The shift of this band to higher energy by ca. 40 cm-1 is typical.12,13,25 The transient band to higher energy is weak, for two reasons: (i) It overlaps with the bleach of the weak highest energy ν(CN) band at ca. 2100 cm-1. (ii) The transient oxidation of the Ru center will diminish the dipole of the C-N bond, which is in the sense Cδ--Nδ+, reducing the oscillator strength for the vibrational transition. The TRIR difference spectra of the dinuclear complex Na4[2] in D2O at different time delays are shown in Figure 6b. Interpretation of these spectra is aided by comparison with the homodinuclear complex [{Ru(CN)4}2(µ-bpym)]4- in which the 3MLCT excited state was best described as having a valencelocalized electronic structure [(NC)4RuII(bpym•-)RuIII(CN)4]4on the IR time scale:13 two sets of ν(CN) transient bands were apparent, one at higher energy corresponding to the transiently oxidized Ru(III) center and one at lower energy corresponding to the “spectating” Ru(II) center which was made more electronrich by having electron density pushed toward it. The TRIR difference spectra of [2]4- display a similar set of features. The main ground-state ν(CN) band at 2070 cm-1 is bleached, and two new transient bands are present, a weak one to higher energy and an intense one to lower energy. These all decay synchronously with a time constant of 56 ns. The high-energy transient is again weak and can be seen as a small positive-going feature at 2115 cm-1 [again, partially obscured by the bleach of the highest energy ground-state ν(CN) band]sa shift to higher energy of 45 cm-1 corresponding to the transiently oxidized Ru(III) center. The second transient band is very intense and covers the region from 1950 to 2100 cm-1; it overlaps heavily with the bleach of the ground-state ν(CN) band which cuts into it generating a double-peaked appearance which is misleading. This overlap means that estimating the position of this intense ν(CN) transient band in these experiments is difficult, but the shift to lower energy is unlikely to be more than ca. 20 cm-1. This transient band corresponds to the spectating Ru(II) center, for which the ν(CN) band is expected to be shifted to lower energy because the bridging ligand has become electron-rich in the MLCT excited state, which will in turn make the Ru(II) center more electron-rich and increase the Ru[d(π)] f CN(π*) back-bonding. In addition, the electron-richness of the Ru(II) center will augment the Cδ--Nδ+ dipole compared to the ground state, which accounts for the high intensity of this transient band. The smaller shift to lower energy of this transient compared to the bipyrimidine-bridged complex [{Ru(CN)4}2(µ-bpym)]4(shift of ca. 40 cm-1) is consistent with the larger size of the conjugated π-system in the HAT ligand compared to bpym, which allows the excited electron to delocalize over a greater spatial region such that the spectating Ru(II) center is less affected by the electron density on the bridging ligand. It is also noteworthy that the excited-state lifetime of [2]4- under 11496 J. AM. CHEM. SOC.

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Figure 7. Time-resolved IR difference spectra of [3]6- in D2O each measured at several time intervals after excitation. The key shows the time delay after excitation (in nanoseconds) at which each spectrum was recorded.

these conditions (56 ns) is remarkably longer than that of [{Ru(CN)4}2(µ-bpym)]4- (1.2 ns). Others have noted that delocalization of the ligand-based electron in MLCT excited states of metal-polypyridyl complexes results in substantial increases in excited-state lifetimes, and a similar effect appears to be operating here.26 The greater rigidity of HAT compared to bpym (which can twist about its central C-C bond) may also have a role in inhibiting some deactivation pathways associated with molecular vibrations. As with [{Ru(CN)4}2(µ-bpym)]4-, the MLCT excited state of [2]4- is clearly localized on the IR time scale and can be described as [(NC)4RuII(HAT•-)RuIII(CN)4]4with separate transient IR signals for spectroscopically distinct Ru(II) and Ru(III) centers.27 The TRIR difference spectra of the trinuclear complex Na6[3] in D2O at different time delays are shown in Figure 7. The behavior of [3]6- is generally similar to that of [2]4- although there are some significant differences. The excited-state spectrum shows a strong bleach of the ground-state ν(CN) band at 2070 cm-1 and weaker bleaches at 2055 and 2100 cm-1, which correspond to two weaker ground-state ν(CN) bands. The two lower energy bleaches are superimposed on a broad, intense transient band which covers the region from 2000 to 2100 cm-1. The bleach and transient bands decay synchronously with a time constant of 18 ns. The overlap of ground-state bleach and excited-state transient bands means that in these experiments the maximum of the transient band cannot be accurately determined, but from Figure 7 it appears that that it will be shifted to slightly lower energy than the ground-state bleach, in agreement with what we observed for [2]4-. Thus, this intense transient band can be ascribed to the main ν(CN) band of the spectating Ru(II) centers which are not involved in formation of the MLCT excited state but which are rendered more electronrich because of the excited-state electron localized on the HAT ligand. The weak higher energy transient band associated with the transiently oxidized Ru(III) center, which is expected at ca. 2110 cm-1, is not apparent. This can be understood with reference to the ground-state FTIR spectrum of [3]6-, which shows that the highest energy ν(CN) band, at 2100 cm-1, is quite intense in this complex. This results in a strong bleach signal in the TRIR at 2100 cm-1 which will mask the expected weak transient band in this region. However, it is clear that the MLCT excited state of this complex can also be described as valence-localized on the IR time scale, i.e., [{(NC)4RuII}2(HAT•-)(26) Harriman, A.; Ziessel, R. Chem. Commun. 1996, 1707. (27) (a) Plummer, E. I.; Zink, J. I. Inorg. Chem. 2006, 45, 6556. (b) Lockard, J. V.; Zink, J. I.; Konradsson, A. E.; Weaver, M. N.; Nelsen, S. F. J. Am. Chem. Soc. 2003, 125, 13471.

Cyanoruthenate Complexes of Hexaazatriphenylene

RuIII(CN)4]6-; a delocalized system with all three metal centers equivalent and having an average oxidation state of 2.33+ would not give a low-energy shifted transient band consistent with spectating Ru(II) centers. The lower lifetime of 18 ns for the MLCT excited state of [3]6- compared to [1]2- and [2]4- is consistent with the lower excited-state energy (the “energy-gap law”),28 although this cannot be used to explain simply why [2]4- has a longer-lived excited state than [1]2. To conclude this section, these TRIR studies have highlighted two important points. First, although the 3MLCT excited states of these complexes are essentially nonluminescent in aqueous solution at room temperature, they are relatively long-lived (tens of nanoseconds) which implies that they could be effective energy donors in supramolecular assembliessan interesting prospect given their remarkable light-harvesting properties as shown in their absorption spectra. Second, for dinuclear [2]4and trinuclear [3]6-, the 3MLCT excited states are localized on the IR time scale with spectroscopically distinct Ru(II) and Ru(III) centers generating two sets of transient bands in the ν(CN) region. Structures of Coordination Networks with Ln(III) Cations. We prepared and structurally characterized coordination networks using the three cyanoruthenate anions with various Ln(III) cations, for two reasons. First, we were interested to evaluate the ability of the complexes to generate structurally novel highly connected networks. The use of Ln(III) cations, which can tolerate higher coordination numbers than d-block ions and therefore are more likely to generate high-dimensionality networks, therefore seemed a sensible choice.29 Second, many of the Ln(III) ions have low-energy f-f excited states which show luminescence in the near-infrared region, and we were interested to examine the ability of [1]2-, [2]4-, and [3]6-swhich absorb light very strongly in the visible region and have reasonable excited-state lifetimessto act as sensitizers of Ln(III)-based near-IR luminescence following d f f energy transfer, as we have shown in other cyanide-bridged d/f networks.4a,5a Crystalline salts were prepared by slow evaporation of aqueous solutions containing [1]2-, [2]4-, or [3]6- (as their Na+ salts) and Ln(III) cations as their chloride or nitrate salts. Ln(III) salts of [1]2- have two different types of solid-state structure according to the size of the Ln(III) cation. When Ln ) Nd, Sm, Eu, and Gd, the crystal structures [{Ln(H2O)5}2{1}3·nH2O]∞ (Ln ) Nd, n ) 18; Ln ) Sm, n ) 11; Ln ) Eu, n ) 16; Ln ) Gd, n ) 16) are essentially identical and crystallographically isomorphous; the structure for Ln ) Nd is in Figures 8 and 9 (see also Table 3). The structure consists of polymeric twodimensional sheets in the crystallographic xy plane. There are two independent types of [1]2- unit, containing Ru(1) (with one such unit colored purple in Figure 9) and the other containing Ru(2), which lies on a 2-fold rotation axis; one such example is colored cyan in Figure 9. Each [1]2- unit based on Ru(1) coordinates one if its four cyanide groups to Nd(1), via N(36), and one of the available bidentate sites on the HAT ligand to another Nd(1), via N(24) and N(14). Each [1]2- unit based on Ru(2) also interacts with two Nd(1) ions but in both cases via cyanide bridges involving N(64) and its symmetry-equivalent (28) Caspar, J. V.; Kober, E. M.; Sullivan, B. P.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104, 630. (29) Hill, R. J.; Long, D. L.; Chanpmess, N. R.; Hubberstey, P.; Schro¨der, M. Acc. Chem. Res. 2005, 38, 335.

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Figure 8. Asymmetric unit of the structure of [{Nd(H2O)5}2{1}3·18H2O]∞ with additional atoms from adjacent asymmetric units to complete the coordination spheres around the metal centers included.

Figure 9. Two-dimensional network structure of [{Nd(H2O)5}2{1}3· 18H2O]∞, with two of the crystallographically independent [1]2- units colored separately (purple and cyan) for clarity (Nd, green; Ru, orange; O, red; N, blue; C, black).

N(64A). Each Nd(1) center is 9-coordinate, from five water ligands, two cyanide bridges [one to Ru(1) and one to Ru(2)], and a HAT-diimine site from another Ru(1) unit. Since each Nd(III) cation interacts with three [1]2- dianions and each [1]2dianion interacts with two Nd(III) cations, the necessary 3:2 ratio of [1]2-:Nd(III) components results. All of the cyanide ligands that are not involved in coordination to Nd(III) are instead hydrogen-bonded to water molecules as shown by numerous nonbonded N···O separations of